1. Technical Field
The disclosure relates to a scattered light measurement apparatus for measuring information on an internal structure of an object to be measured, as an amount of scattering and absorption of light.
2. Related Art
Conventionally, backscattered light returned from a relatively weak scattering medium such as a biological tissue is detected as interference enhanced light according to a degree of spatial coherence of light illuminating the scattering medium (see Non-Patent Literature 1). A spectroscopic information measuring technique that uses this phenomenon is called low-enhanced backscattering spectroscopy (LEBS) and characteristics of interference patterns with respect to a scattering mean free path (an inverse of a scattering coefficient) ls* in the scattering medium have been well studied (see Non-Patent Literature 2). This scattering mean free path ls* has a correlation with an internal structure change in the scattering medium, and is used to detect a minute change in a tissue structure such as that found in cancer in an early stage. It has also been known that discrimination of colon cancer is possible using interference patterns of returning scattered light (see Non-Patent Literature 3).
In the LEBS described above, a technique applied to noninvasive measurement in a human body performed through a small diameter probe inserted in an endoscope is known (see Patent Literature 1). In this technique, to obtain an interference pattern, detection fibers are arranged at a plurality of different positions (corresponding to different scattering angles) on a plane where an interference pattern is formed and signals are detected by corresponding detectors.
Furthermore, in the LEBS, detection of backscattered light from a scatterer surface layer under limiting condition is performed, and a detection depth in the scatterer surface layer is controlled by a spatial coherence length. FIG. 13 is a schematic diagram illustrating main elements of a scattered light measurement probe, which is a conventional small diameter probe. A scattered light measurement probe 200 illustrated in FIG. 13 includes an illumination fiber 201 that emits illumination light to an object to be measured (scatterer surface layer 300), a plurality of detection fibers 202a to 202c on which returned light of the illumination light reflected and/or scattered by the object to be measured is incident at different angles, and an optical element 210 provided at tips of the illumination fiber 201 and the detection fibers 202a to 202c. 
The optical element 210 is cylindrically shaped and is formed of a transmissive glass having a specified refractive index. When the measurement is performed, a tip of the optical element 210 comes in contact with the scatterer surface layer 300, so that a distance from the illumination fiber 201 and the detection fibers 202a to 202c to the object to be measured is fixed.
In this case, the detection depth D100 of the scatterer 300 is defined by a spatial coherence length Lsc. The spatial coherence length Lsc satisfies a relation of formula (1) below when a length of the optical element 210 in a central axis direction of the cylinder is R, the refractive index of the optical element 210 is n, the diameter of the illumination fiber 201 is ρ, and the wavelength of the light emitted from the illumination fiber 201 (light source) is λ.Lsc=λR/πρn  (1)
In the scattered light measurement probe 200, R and ρ are set so that the spatial coherence length Lsc becomes of a sufficiently smaller value than the scattering mean free path ls*.